1. Field of the Invention
The present invention elates to a surface mount quartz crystal unit, and more particularly to a surface mount crystal unit which is constructed to suppress noise generated thereby.
2. Description of the Related Art
Quartz crystal units are incorporated in oscillators as a time or frequency reference source. Particularly, surface mount crystal units are small in size and weight, and hence are widely used in various portable communications devices, for example. In recent years, specifications that surface mount crystal units are required to fulfill have been becoming stricter, and there has been a demand for surface mount crystal units which produce less noise and particularly have better phase noise characteristics.
As shown in FIGS. 1A and 1B, a surface mount crystal unit comprises casing 1 having a recess defined therein, quartz crystal blank 2 disposed in the recess, and cover 3 placed on casing 1 over the recess and hermetically sealing crystal blank 2 in the recess. Casing 1 has a substantially rectangular planar shape, and is of a laminated structure of ceramics which comprises substrate 4 for surface-mounting and frame wall 5 mounted on substrate 4. Substrate 4 has a substantially rectangular planar shape, and frame wall 5 has an opening which serves as the recess in casing 1. A pair of connecting electrodes 6 for connection to crystal blank 2 is formed on respective opposite sides of one end of an inner bottom surface of the substantially rectangular recess. A pillow member 7 is mounted on the other end of the inner bottom surface of the recess. The connecting electrodes 6 are electrically connected to mounting electrodes 8 disposed on an outer surface of casing 1 via the interface across which substrate 4 and frame wall 5 are laminated and through holes (not shown) that are defined in substrate 4.
As shown in FIG. 2, crystal blank 2 comprises a substantially rectangular AT-cut quartz crystal blank. Excitation electrodes 9 are formed respectively on the principal surfaces of crystal blank 2, and extension electrodes 10 extend from respective excitation electrodes 9 to opposite sides of one end of crystal blank 2. Extension electrodes 10 are folded back a short distance over the other principal surfaces across the end of crystal blank 2.
The opposite sides of the end of crystal blank 2 are bonded to respective connecting electrodes 6 on the inner bottom surface of the recess in casing 1 by electrically conductive adhesive 11, thus electrically and mechanically connecting the opposite sides of the end of crystal blank 2 to connecting electrodes 6. The other end of crystal blank 2 is placed on pillow member 7, but is not fixed to pillow member 7.
In the case where crystal blank 2 has a beveled or convex cross-sectional shape, pillow member 7 serves to prevent the vibrating region of crystal blank 2 where excitation electrodes 9 are disposed, from contacting the inner bottom surface of the recess. Casing 1 with pillow member 7 is also used as a common component in the case where crystal blank 2 is in the form of a flat plate.
However, the surface mount crystal unit of the above construction may have its vibrating characteristics lowered when the other end of crystal blank 2 is brought into contact with pillow member 7 due to external vibrations. For example, if the surface mount crystal unit is incorporated in a cellular phone, then a noise component is added to the vibrating frequency of the crystal unit, impairing the phase noise characteristics while the cellular phone is in operation.
The other end of crystal blank 2 is disposed in abutment against or in the vicinity of pillow member 7. If a vibration source such as a motor or the like is incorporated in the portable device, then other end of crystal blank 2 is brought into intermittent contact with pillow member 7. Low-frequency vibrating sound is propagated to the other end of crystal blank 2, and then the low frequency components are added to the vibrating frequency component. As a result, the phase noise characteristics of the surface mount crystal unit is impaired.
To solve the above problem, Japanese laid-open patent publication No. 2002-84160 (JP, P2002-84160A) discloses a surface mount crystal unit that is free of a pillow member. As shown in FIG. 3A, the disclosed surface mount crystal unit has ridge 12 as a pivot for crystal blank 2 is disposed near connecting electrodes 6 on the inner bottom surface of the recess in casing 1. Electrically conductive adhesive 11 is applied to a pair of connecting electrodes 6, and the opposite sides of one end of crystal blank 2 are placed on electrically conductive adhesive 11. At this time, the other end of crystal blank 2 abuts against the inner bottom surface of the recess. FIG. 3B is a plan view of the surface mount crystal unit with a cover omitted from illustration. Each of connecting electrodes 6 is of a rectangular shape, and ridge 12 is integrally formed with connecting electrodes 6 along one side thereof. Ridge 12 extends in the direction of the shorter sides of crystal blank 2. Ridge 12 is usually made of a metal material. In FIG. 3B, crystal blank 2 is shown by the broken lines for illustrative purposes.
When electrically conductive adhesive 11 is set or cured, the other end of crystal blank 2 is lifted under shrinking forces of electrically conductive adhesive 11 as indicated by the arrows P, Q in FIG. 3C, creating and maintaining a spacing between crystal blank 2 and the inner bottom surface of the recess in casing 1. Thereafter, cover 3 is placed over the recess, thus completing the surface mount crystal unit.
However, since ridge 12 is made of the metal material, the surface mount crystal unit suffers the following problems: Electrically conductive adhesive 11, which comprises an adhesive made of an organic material mixed with an electrically conductive filler, generally has a less tendency to adhere to metal than an insulating material, i.e., ceramics. When a mechanical shock is applied to the completed surface mount crystal unit, vertically swinging the other end of crystal blank 2, electrically conductive adhesive 11 is liable to peel off the interface between itself and ridge 12 due to stresses concentrated between electrically conductive adhesive 11 and ridge 12 which functions as the pivot for crystal blank 2, as indicated by the arrow A in FIG. 4 which shows the fixed portion of crystal blank 2 at an enlarged scale. Similarly, electrically conductive adhesive 11 is liable to peel off connecting electrodes 6.